1. Field of the Invention
This invention relates to a noise reduced photon detector, and in particular (but not exclusively) to an infrared detector.
2. Discussion of Prior Art
Infrared detectors of semiconductor material such as cadmium mercury telluride (CMT) are well known. Conventional types are normally cooled to below 200K or 80K for operation at a radiation wavelength in the 3-5 xcexcm band or the 8-12 xcexcm band respectively. Cooling is needed to provide for the detector""s semiconductor material to be in an extrinsic regime, ie to reduce the intrinsic contribution to conduction, to increase sensitivity and reduce Auger-generated noise. Broadly speaking, there are three important conduction regimes for a semiconductor: unsaturated extrinsic, saturated extrinsic and intrinsic, which occur at low, moderate and high temperature respectively. Unsaturated extrinsic conduction is due to one type of carrier activated from impurities, not all of which are ionised. The saturated equivalent is similar except that all impurities are ionised. In the intrinsic regime, conduction has a substantial contribution from valence band to conduction band transitions producing both types of carrier, ie electron-hole pairs. There is an intervening transition region between the second and third regimes where conduction is a combination of extrinsic and intrinsic.
More recently, as set out in U.S. Pat. No. 5,016,073,it has proved possible to reduce the intrinsic contribution to conduction in a semiconductor detector by designing it to employ the phenomena of exclusion and/or extraction to reduce the carrier concentration. An excluding contact allows majority carriers to flow in one direction but inhibits the reverse flow of minority carriers. An extracting contact removes minority carriers which diffuse to it. In consequence, a semiconductor region between two such contacts becomes depleted of minority carriers when electrically biased, and charge neutrality considerations dictate that the majority carrier concentration fall to the same extent. The combination of these two effects is to reduce the intrinsic contribution to conduction, simulating the effect produced by cooling and allowing detector operation at higher temperatures either uncooled or with a reduction in cooling requirements.
It is an object of this invention to provide a detector with improved noise characteristics.
The present invention provides a noise-reduced photon detector including an array of detector elements characterised in that it includes at least one of the following:
(a) isolating means for isolating each element from photons emitted from other elements and other regions of the detector,
(b) the elements being arranged to exhibit negative luminescence and thereby reduced photon emission, and
(c) a structure arranged to exhibit negative luminescence and to absorb photons which would otherwise propagate to detector elements and give rise to photon noise.
It has been found that, in detectors operated at ambient or near ambient temperatures and arranged to suppress Auger-generated noise, radiative noise becomes important, and the invention provides the advantage of reducing it.
In a preferred embodiment, a detector of the invention preferably includes both isolating means for isolating each element from photons emitted by other elements and detector elements arranged to exhibit negative luminescence and thereby reduced photon emission.
Each detector element may comprise an extrinsic active region. Alternatively, each active region may exhibit intrinsic conductivity when unbiased. Each active region may be sandwiched between two regions of wider bandgap arranged to reduce its minority carrier concentration. The wider bandgap regions may be of opposite conductivity type to one another. The active region and wider bandgap regions may in turn be sandwiched between two further regions of wider bandgap than the active region and of higher doping than the other regions.
Each detector element may be a semiconductor diode arranged to exhibit at least one of carrier exclusion and extraction under reverse bias to reduce an intrinsic contribution to carrier concentration in an active element region.
The isolating means may incorporate a mirror arranged to reflect away from detector elements photons not received directly from an scene imaged thereon. It may incorporate a cold shield arranged to screen the detector elements from unwanted radiation, or alternatively internally tapering reflectors such as Winston cones arranged to concentrate radiation from an observed scene upon respective detector elements and to screen them as aforesaid. Alternatively, a photonic bandgap structure may be arranged to restrict unwanted photon propagation between detector elements.